1. Field of the Invention
The present invention relates to a liquid crystal projection display having excellent relative corner illuminance characteristics.
2. Description of the Prior Art
Referring to FIG. 1a showing a prior art liquid crystal projection display of a one-panel one-lens type, there are shown a surface light source 1, a Fresnel collimator lens 2, a liquid crystal panel 3 of about 250 mm in diagonal length, 200 mm in width, 150 mm in height and about 2 mm in thickness, a converging Fresnel lens 4, a projection lens 5 and a screen 6. In FIG. 1a, .beta. and .beta.' are angles of view and .omega. (rad) is the angle of divergence of light rays passing through the liquid crystal panel 3. An image formed on the liquid crystal panel 3 is focused on the screen 6 by the projection lens 5. As is generally known, the light modulating characteristics of the generally known TN (twisted nematic) liquid crystal panel are dependent on the inclination of the transmitted light rays. The mode of dependence of the light modulating characteristics on the incidence angle is anisotropic; light rays diverge in a narrower directivity angle in the direction of the plane of polarization and diverges in a wider directivity angle in a direction perpendicular to the plane of polarization. In an ordinary TN liquid crystal panel, the direction of the narrower directivity angle corresponds to a vertical direction, and the direction of the wider angle corresponds to a horizontal direction. Accordingly, an optical system capable of reducing the angle of divergence in the direction of the narrower directivity angle of the liquid crystal panel must be used to enable the liquid crystal panel to function in satisfactory light modulating characteristics, i.e., to form an image of a high contrast. It is empirically known that the contrast C is approximately equal to 6/.omega..sup.2. When reproducing a natural picture, the contrast C must be 100 or above to reproduce the natural picture in high contrast. Accordingly, the angle .omega. of divergence must be about 0.25 rad or below.
The liquid crystal panel 3 has a surface provided with 640.times.480 pixels of about 0.3 mm square in area. FIG. 1b shows the shape of a pixel for a monochromatic liquid crystal panel, in which a shaded portion is the blackened area that does not transmit light rays. FIG. 1c shows the shape of one pixel trio for reproducing color images, in which a shaded portion is a blackened area, and R, G and B indicate red, green and blue pixels. The ratio of the effective area of to the total area of the pixel of FIG. 1b is about 64%, and that of the pixel trio of FIG. 1c is about 32%. The relative corner illuminance (RCI) of the prior art is shown in FIG. 2a, in which the converging angle .alpha. of the light rays emitted by the surface light source 1 of FIG. 1a is measured to the right on the horizontal axis, and the relative corner illuminance on the plane of incidence of the Fresnel collimator lens 2 is measured upward on the vertical axis. The distribution of relative corner illuminance on the Fresnel collimator lens 2 is substantially the same as that on the liquid crystal panel 3. As is generally known, the distribution of relative corner illuminance conforms to the Cos.sup.4 law. FIG. 2b is a graph showing the variation of the light capturing efficiency E(.alpha.) of the Fresnel collimator lens 2 with the angle .alpha. of convergence. As is generally known, the variation of the light capturing efficiency E(.alpha.) conforms to the Sin.sup.2 law. As is obvious from FIGS. 2a and 2b, whereas E(.alpha.)=100% when and RCI=0% when .alpha.=.pi./2, E(.alpha.)=50% and RCI=25% when .alpha.=.pi./4. The relative corner illuminance needs to be about 50% to form an image in a satisfactory quality. Accordingly, as is obvious from FIG. 2a, the converging angle .alpha. must be about 0.57 rad or below and, consequently, the light capturing efficiency E(.alpha.) is reduced inevitably to 29% as shown in FIG. 2b. Thus, it is difficult for the prior art to improve the relative corner illuminance without reducing the light capturing efficiency E(.alpha.).
As mentioned above, the light capturing efficiency conforms to the Sin.sup.2 law and the relative corner illuminance conforms to the Cos.sup.4 law when a single flat collimator lens and a single small surface light source disposed at the focal point of the flat collimator lens are used in combination. Therefore, there is a reciprocal relation between the light capturing efficiency and the relative corner illuminance that the sum of the square root of the relative corner illuminance and the light capturing efficiency is always 1 or not greater than 1.
It is possible to prove that the reciprocal relation applies also to the combination of a single paraboloidal collimator mirror and a single point light source disposed at the focal point of the paraboloidal collimator mirror. The ratio of collimated light, namely, the light capturing efficiency is equal to sin.sup.2 0.5.alpha. and the relative corner illuminance is equal to cos.sup.4 0.5.alpha., where .alpha. is convergence angle. Accordingly, the sum of the square root of the relative corner illuminance and the light capturing efficiency is equal to 1.
FIG. 3a shows a portion of the converging Fresnel lens 4 of FIG. 1a. In FIG. 3a, the incident light rays 9 are represented by a continuous line, the normal outgoing light rays 10 are represented by continuous line and ghost light rays 12, namely, undesired interference light rays, are represented by broken lines. Some of the incident light rays 9 reflected by the plane 4' of exit of the converging Fresnel lens 4, namely, ghost light rays, are reflected by the plane 11 of incidence of the converging Fresnel lens 4 in total reflection. The ghost light rays 12 form a ghost image on the screen.
A means for reducing ghost interference disposes the converging Fresnel lens 4 in a reverse position as shown in FIG. 3b. However, when the converging Fresnel lens 4 is disposed in a reverse position, some of the incident light rays 9 falling on shaded portions 9" do not travel in the correct direction of outgoing light rays 10, which reduces the relative corner illuminance. Thus, it is difficult for the prior art to increase the relative corner illuminance without entailing ghost interference.
Although the ghost interference caused by the ghost light rays produced by the converging Fresnel lens 4 disposed behind the liquid crystal panel 3 as shown in FIG. 1a is conspicuous, the ghost light rays produced by the Fresnel collimator lens 2 disposed before the liquid crystal panel 3 do not cause problems because the ghost light rays travel outside the field of view of the projection lens 5.
As is apparent from the foregoing description, it is difficult for the prior art to prevent the reduction of light capturing efficiency or undesirable ghost interference and, at the same time, to enhance relative corner illuminance.